Improve #include handling for JS library files

Files references via #include can now either relative to the including
file or come from the system library.  Previously only relative paths
were allowed.

Also we now give a nicer error message when the file is not found.

src/parseTools.mjs

@@ -9,6 +9,7 @@
  */
 
 import * as path from 'node:path';
+import {existsSync} from 'node:fs';
 
 import {
   addToCompileTimeContext,
@@ -19,6 +20,7 @@ import {
   runInMacroContext,
   setCurrentFile,
   warn,
+  srcDir,
 } from './utility.mjs';
 
 const FOUR_GB = 4 * 1024 * 1024 * 1024;
@@ -40,6 +42,24 @@ export function processMacros(text, filename) {
   });
 }
 
+function findIncludeFile(filename, currentDir) {
+  if (path.isAbsolute(filename)) {
+    return existsSync(filename) ? filename : null;
+  }
+
+  // Search for include files either relative to the including file,
+  // or in the src root directory.
+  const includePath = [currentDir, srcDir];
+  for (const p of includePath) {
+    const f = path.join(p, filename);
+    if (existsSync(f)) {
+      return f;
+    }
+  }
+
+  return null;
+}
+
 // Simple #if/else/endif preprocessing for a file. Checks if the
 // ident checked is true in our global.
 // Also handles #include x.js (similar to C #include <file>)
@@ -125,11 +145,12 @@ export function preprocess(filename) {
             if (includeFile.startsWith('"')) {
               includeFile = includeFile.substr(1, includeFile.length - 2);
             }
-            // Include files are always relative to the current file being processed
-            if (!path.isAbsolute(includeFile)) {
-              includeFile = path.join(path.dirname(filename), includeFile);
+            const absPath = findIncludeFile(includeFile, path.dirname(filename));
+            if (!absPath) {
+              error(`${filename}:${i + 1}: file not found: ${includeFile}`);
+              continue;
             }
-            const result = preprocess(includeFile);
+            const result = preprocess(absPath);
             if (result) {
               ret += `// include: ${includeFile}\n`;
               ret += result;

src/utility.mjs

@@ -233,11 +233,13 @@ export function read(filename) {
 // Use import.meta.dirname here once we drop support for node v18.
 const __dirname = url.fileURLToPath(new URL('.', import.meta.url));
 
+export const srcDir = __dirname;
+
 // Returns an absolute path for a file, resolving it relative to this script
 // (i.e. relative to the src/ directory).
 export function localFile(filename) {
   assert(!path.isAbsolute(filename));
-  return path.join(__dirname, filename);
+  return path.join(srcDir, filename);
 }
 
 // Anything needed by the script that we load below must be added to the

test/test_other.py

@@ -4922,6 +4922,22 @@ def test_jslib_errors(self):
     self.assertContained('error_in_js_libraries.js:5: #error This is an error string!', err)
     self.assertContained('error_in_js_libraries.js:7: #error This is a second error string!', err)
 
+  def test_jslib_include(self):
+    create_file('inc.js', '''
+    let MY_VAR = 10;
+    ''')
+    create_file('foo.js', '''
+    // Include a file from system directory
+    #include "arrayUtils.js"
+    // Include a local file.
+    #include "inc.js"
+    ''')
+    self.run_process([EMCC, test_file('hello_world.c'), '--js-library', 'foo.js'])
+
+    delete_file('inc.js')
+    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library', 'foo.js'])
+    self.assertContained('foo.js:5: file not found: inc.js', err)
+
   def test_postjs_errors(self):
     create_file('post.js', '#preprocess\n#error This is an error')
     err = self.expect_fail([EMCC, test_file('hello_world.c'), '--post-js', 'post.js'])